This invention relates to semiconductor lasers fabricated after device growth utilizing impurity induced disordering (IID) and more particularly the fabrication and design of multi-emitter semiconductor laser arrays having minimal electrical and thermal crosstalk and high efficiency useful for high speed raster output scanners (ROS) and laser printing applications.
The ability to fabricate closely spaced independently addressable laser sources is important for many applications such as optical disk technology, laser printing, optical interconnection and fiber optic communications. It is often desirable to have the laser elements of a laser array in as close proximity as possible in order to simplify optical system design. For optical interconnections, and especially when spacing between laser elements is only a few microns, it is highly desirable to mount the devices with their p-side up in order to simplify the separation of electrical connection to the laser devices. However, this places constraints on device performance in order to achieve CW operation. Previous attempts have been made to provide separate contacting of laser elements of such devices but these devices were not capable of CW operation. In addition, the optical and carrier confinement was insufficient to prevent coupling and phase locking between sources.
Acceptable CW performance has been obtained in p-side up configuration with etch and regrown buried heterostructure lasers, but reliability and yield remain key issues in production of high density laser arrays by this technique.
Single emitter lasers generally of the III-V material regime, e.g., GaAs/GaAlAs, have a designed higher refractive index cavity which is formed between laterally adjacent regions of comparatively lower refractive index. It is known to produce such optical cavities by means of nonplanar growth mechanisms, such as a channel or mesa in the laser substrate or by means of impurity induced disordering (IID) as exemplified in U.S. Pat. No. 4,378,255 to Holonyak. As taught in this patent, a semiconductor structure containing a quantum well feature, such as a multiple quantum well, undergoes compositional disordering due to impurity diffusion. Diffusion of an impurity into spatially separated regions of the quantum well feature will cause an intermixing of Al and Ga in the well feature so that the average refractive index through the region of these layers subjected to disordering by diffusion will have a lower index of refraction compared to undisordered regions including the central region between the designated spatially separated regions. Thus, the central region may be utilized as an optical waveguide cavity for lasing and/or light propagation.
It has been shown that silicon impurity induced disordering (Si-IID) technology is capable of producing low threshold buried heterostructure lasers with power conversion efficiencies on the order of 50% at few milliwatt power levels. This high level of performance permits these types of devices to be mounted p-side up and CW operated. In addition, it has been shown that laser arrays of this type with center-to-center separations as low as 4 .mu.m with a single contact addressing electrode exhibit a high degree of uniformity and do not exhibit phase locked operation as a result of the strong refractive index waveguiding mechanism provided via Si-IID.
The use of laser or LED arrays for laser printers having flying spot scanners or raster output scanners (ROS) have been suggested previously, as exemplified in U.S. Pat. Nos. 4,445,125; 4,474,422 and 4,796,964, assigned to the same assignee as herein, because of their small size, low power requirements, longevity, ease of fabrication, low cost and sensitivity in the infrared spectra for exposure of infrared photoreceptors to create or write a latent electrostatic image on the charged photoreceptor surface. There is also the advantage of scanning simultaneously more than one scan line at a time with two or more beams from a monolithic semiconductor laser source, as exemplified in U.S. Pat. Nos. 4,474,422 and 4,796,964 to concurrently provide improved resolution and higher speed in a polygon ROS printer. The use of solid state light source eliminates the difficulties associated with complicated optical systems required for beam splitting and beam alignment when a single gas laser source is employed. The small, compact size of the semiconductor laser is particularly suited for this application.
However, a major complication with the semiconductor laser is the difficulty in fabricating the monolithic laser sources in sufficiently close proximity such that two adjacent scan lines may be simultaneously written. As a result, most systems contemplating the use of semiconductor multiple emitter sources utilize an interlaced scanning system wherein nonadjacent scan lines at the imaging bearing surface are written simultaneously, e.g. every third or fourth scan line. This is not a preferred solution because the utilization of nonadjacent scan lines results in nominally adjacent scan lines being written in different real time frames resulting in extreme sensitivity of print quality to temporal stability of the scanning system and the photoreceptor velocity. Vibrations in the system emanating from various mechanical system components, therefore, have a major adverse impact on print quality. In addition, the interlace systems used to write the scan lines requires data buffers and adds to the complexity of the drive electronics, a complication that increases rapidly in impact as the number of independent write sources increases.
It is a principal object of this invention to use IID in providing high density arrays of independently addressable semiconductor laser sources, particularly useful in ROS and laser printing applications and, in particular, provide two write sources that concurrently are spatially separated by a sufficiently large distance to concurrently write at least two adjacent scan lines without the need for interlacing, data buffers and complicated optics for beam translation to the image bearing surface in a ROS environment.